Free radical polymerization is a preferred technique in the synthesis of many polymers. One drawback of free radical polymerization is the lack of control over the polymer architecture produced. The type and amount of initiator, temperature, and delayed monomer feeds have all been used to control the final structure and size of the polymer particles.
Living polymers offer some control of the polymer structure. Living polymers are polymers having an active/growing end on a polymer chain (non-terminated polymer chain). Most commonly, living ends are formed by anionic polymerization in non-polar solvent, or involve a capping-mechanism to stop a growing free-radical, then restarting the polymer growth by reversible removal of the cap.
“Low VOC Latex Paints from a Precipitation Polymerization Process”, Clean Prod. Processes, 3 (2001), 5-59 discloses the formation of a methyl methacrylate/butyl acrylate copolymer from a conventional precipitation reaction using n-heptane as the solvent. The resulting dispersion is bimodal. A problem with conventional precipitation polymerization is that conversion rates are generally very low, requiring a relatively expensive procedure to isolate the polymer and recycle monomer.
Chemical trapping agents are known in the art as a means of capping a living polymer radical by a reversible means, then uncapping the living radical for later continued polymerization. Unfortunately such a method requires additional purification costs to remove the trapping agent from the product.
U.S. Pat. No. 5,173,551 and “Studies of the Polymerization of Methacrylic Acid via Free-Radical Retrograde Precipitation Polymerization Process”, J. Applied Polymer Science, Vol. 62, 2039-2051 (1996) describe the use of a free-radical retrograde polymerization process as a means of controlling the polymer molecular weight distribution. In this process a monomer mixture in a solvent is initiated by a solvent-soluble free radical initiator to produce polymer radicals that precipitate into polymer-rich phases in a solvent. The above patent also addresses the capability of FRRPP to control the activity of polymer radicals and use them to produce copolymers. Additionally it provides a detailed description of the original process, a possible apparatus for running it, and ways of regulating the rate of reaction through changing temperature and pressure and adding or removing monomers or inert diluents.
Surprisingly, it has been found that a dispersion containing polymer particles containing multiple living polymer radicals can be formed using a free radical retrograde precipitation process. The free radical containing dispersion can be further reacted with additional amounts of the same monomer to form higher molecular weight polymer, or reacted with a second monomer to form a block copolymer dispersion having a controlled structure. This method can be used to produce block copolymers having a controlled morphology and molecular weight. The process can also produce block copolymer emulsions of monomers previously not capable of forming stable emulsion polymers.